The increasing requirement of quality control and efficiency in assembly plants such as manufacturing plants has resulted in the development of sophisticated assembly tools. For example, with regard to the tightening of joints, threaded fasteners, such as nuts, screws or bolts, often have to be rotated a number of turns until a desired tightening force finally has been reached.
The strength of such joints is related to the force by which the fastener holds the two (or more) joint parts together. Therefore, it is of major importance that fasteners of such joints are tightened to such extent that it can be ensured that required tightening force levels are reached. However, even though the sophisticated tools used today provide various methods of ensuring that a desired minimum tightening force of a joint is also in fact reached during the tightening process, e.g. by measuring the torque applied by the tool and the duration of the torque in terms of angular rotation, uncertainties regarding the actual tightening force that has been reached still exist, e.g. due to the fact that the friction between the fastener and the one or more components being joined can vary substantially from joint to joint.
This problem has been addressed by various more or less sophisticated solutions. For example, the dimension of the fastener actually being used can be increased with respect to the theoretical requirement of the dimension of the fastener, thereby ensuring that even if the fastener of larger dimension is not tightened to maximum tightening force, it can still be ensured that the fastener is tightened at least to the extent that is required by the particular design.
According to another solution, the tightening of the fastener is followed by a measurement of the elongation the fastener is subject to during the fastening process. Such measurement is often carried out by a device utilizing ultrasonic technology, and the tightening force can be calculated from the elongation of the fastener resulting from the tightening process. Such use of ultrasonic technology requires that the (length of the) fastener is first measured beforehand, i.e. when the fastener is still in an unstressed state, and then after the fastening process is finished in order to determine the elongation.
Such ultrasonic measurement, however, requires acoustic contact between the transducer that is used to impose ultrasonic sound waves into the fastener, which in turn results in it being difficult to perform measurement during the actual fastening process. The measurement is therefore in general performed after the fastening has been completed, which reduces to a large extent the advantage of using high-speed fastening tools in the fastening process, since the time it takes to perform the ultrasonic measurement inherently will substantially exceed the amount of time it takes to perform the actual fastening.
Therefore, use of ultrasonic measurement is sparsely used in situations in which fast assembly is desired and/or required, and instead being used in situations where the tightening force is of greater importance that the speed at which the tightening is being carried out.
Consequently, there exists a need for a method that allows ultrasonic measurement during a fastening process, without substantially affecting the assembly time.
Further, ultrasonic measurements are useful not only in fastening processes but also various other kinds of situations, e.g. in determination of inhomogeneities in materials/objects, and there exists a need for an improved method of obtaining acoustic contact between measurement device and material/object in a convenient manner, while at the same time ensuring satisfactory signal-to-noise-ratios with regard to the received signal.